Oman opposes upgrading the Gulf Cooperation Council to a union of six nations, an idea first mooted by regional heavyweight Saudi Arabia, Muscat's foreign minister said on Saturday.

The union issue is on the agenda of the GCC summit to be held on Tuesday in Kuwait.

"We are against a union," Omani Foreign Minister Yusuf bin Alawi said at the Manama Dialogue in Bahrain.

The annual forum on security is also being attended by senior world officials including British Foreign Secretary William Hague and Pentagon chief Chuck Hagel.

Alawi was speaking after Saudi Assistant Foreign Minister Nizar Madani in a speech called on Gulf states to unify against dangers in the region.

"We will not prevent a union, but if it happens we will not be part of it," Alawi told AFP on the sidelines of the gathering.

If the five other GCC members -- Saudi Arabia, Kuwait, Bahrain, the United Arab Emirates and Qatar -- decide to form a union, "we will simply withdraw" from the new body, he said.

Riyadh's idea of upgrading the GCC was first proposed in 2011 and supported by Bahrain. But because of reservations by some members, it was put on hold.

Kuwait and Qatar have since expressed their backing, but the UAE's position on the proposal is not known.

Formed in 1981 as a buffer against Shiite-dominated Iran across the Gulf, the mainly Sunni GCC states sit on around 40 percent of proven global crude reserves and around 25 percent of natural gas deposits.